US Population Pyramid Question

Factual Questions
1

I noticed a discrepancy in US Population figures that I can’t quite explain:

  1. The # of births was** increasing from 1976-1990 and also 1997-2007** (let’s call them Early Millennials and Late Millennials):

https://2.bp.blogspot.com/-WIiWAukATG4/WVZ-zIwLjQI/AAAAAAAArkM/CX5QWxblPIQgK2BhMnh3HIQLhSyDvIV6gCLcBGAs/s1600/USBirths2016.JPG

You can also check this on Page 3 of this CDC PDF: https://www.cdc.gov/nchs/data/nvsr/nvsr67/nvsr67_01.pdf

  1. However, US Population Pyramids show the population **under 20 declining **as of 2015/2016/2017:

also https://www.populationpyramid.net/united-states-of-america/2017/

Why? The Early Millennials make an appearance in the population pyramid, but the Late Millennials do not. It’s as if there was no baby boom from 1997-2007, but we know that there was, based on the # of births above.

Any thoughts on this discrepancy?

2

Don’t forget that the population pyramid also includes immigration. The majority of immigrants likely arrive without a lot of under-20 children, as a guess. For the USA, immigration is a significant factor.